Fluorescence microscopy is often used in the fields of molecular biology, biochemistry and other life sciences for analyzing biological molecules, including nucleic acids (DNA, RNA) and proteins (enzymes, antigens, etc.) that have been tagged or labeled with fluorescent probes. One such use is DNA diagnostics, such as for gene detection, in which a DNA sample is deposited on and bound to a glass substrate by means of a chemical binding agent, such as an aminosilane, present on the substrate surface, and a reagent, such as a carbodiimide vapor. The bound DNA on the substrate can then be imaged by fluorescence. The fluorescence of a sample was originally assessed by visual inspection through a conventional microscope, but this manual method has proved time-consuming and costly. Many different high-speed automated fluorescence imaging systems are now available.
An important figure of merit for fluorescence detection and measurement instruments is sensitivity, which is primarily determined by the signal-to-noise ratio (SNR) of the optical imaging system of the instrument. A well-designed imaging system has a signal-to-noise ratio that is limited by its light collection ability and not by internal noise sources. The theoretical SNR of such a system is expressed in terms of the number of photoelectrons at the cathode when using a photomultiplier tube (PMT), which in turn essentially depends upon the number of photons that reach the detector from the area of interest on the sample substrate, the quantum efficiency of the detector, and the number of dark electrons generated by the detector. EQU SNR=S/[S+2B].sup.1/2,
where B is the total background noise and S is the measured signal less B. One obvious approach to increasing SNR, and thereby improving sensitivity, is to reduce background noise. Sources of background noise include specular or diffuse reflection of the fluorescence-stimulating laser light from the sample, autofluorescence of the substrate holding the sample, autofluorescence from the optics in the light path of the optical imaging system, stray light, and dark current of the detector. Stray light reaching the detector can be significantly reduced by proper size and placement of apertures in the imaging system. Both stray light and much of the reflected laser light can be rejected, while passing the fluorescent light, by using dichroic and other spectral filters and beamsplitters in the system. Autofluorescence of the optical elements can be reduced by avoiding use of lens cements in the light path, using glass instead of polymeric lenses, or using curved mirrors instead of lenses wherever possible. Autofluorescence of the substrate can be reduced by using low fluorescence materials, such as an ultrathin or opaque glass substrate. For example, in U.S. Pat. No. 5,095,213 Strongin discloses a plastic slide that is rendered opaque and substantially nonfluorescent with a quantity of black carbon powder in the plastic. Another way of handling autofluorescence is to use a pulsed or modulated excitation and to take advantage of the differences in emission decay rates between background fluorescence and specimen fluorescence, as disclosed in U.S. Pat. Nos. 4,877,965 to Dandiker et al. and 5,091,653 to Creager et al.
In U.S. Pat. No. 5,552,272, Bogart discloses an assay system and method for detecting the presence or amount of an analyte of interest. It includes a test substrate with an optically active surface that enhances the color contrast, i.e. differences in the observed wavelength (or combination of wavelengths) of light from the surface, between the presence and absence of the analyte in a sample applied onto the test substrate. In particular, the substrate may comprise a reflective solid optical support, such as a silicon wafer or metallic (e.g., aluminum) base, with an optical thin film coating thereon. The coating may comprise several layers, including for example an attachment layer on the upper surface of the support, and a receptive layer on the upper surface of the attachment layer containing a specific binding partner for the analyte of interest. The total coating thickness is selected to cause incident light to undergo thin film interference upon reflection, such that a specific color is produced. Specifically, the coating material(s) should have an overall thickness of a quarterwave of the unwanted color to be attenuated so that destructive interference of that color will occur. The substrate therefore has a particular background color, which can then be used as a comparative reference against a different observed color when an analyte of interest is present. Both qualitative visual inspection and quantitative instrumented measurement are suggested. Polarization contrast by means of an ellipsometer is also suggested.
One example of the use to which the Bogart invention has been put by Biostar, Inc. of Boulder, Colo., the assignee of the aforementioned patent, is an optical immunoassay (OIA) diagnostic screening test for the rapid detection (in under 30 minutes) of the presence of specific antigens of infectious pathogens n a sample taken from a patient. Commercial products include test kits for group A and group B streptococci and for chlamydia trachomatis. These particular assays are given as examples in the Bogart patent, are described in package inserts for the corresponding Biostar products and are also described in a number of published articles in medical journals. Briefly, they all rely on direct visual detection of a change in the color of light reflection off of the test substrate due to a physical change in the optical thickness of a molecular thin film coating on the substrate surface which results from binding reactions between an immobilized antibody on the test surface and a specific antigen that may be present in a drop of sample liquid applied to the test surface. The original bare test surface has a thin film thickness that results in a predominant visual background gold color when white light is reflected off of the surface. The antigen-antibody binding reaction that occurs when the specific antigen of interest is present in the applied sample results in an increase in the thin film thickness that causes a corresponding change in the color of the test surface from gold to purple. If on the other hand, the antigen is not present in the sample, no binding takes place, the original thin film thickness remains unchanged and the test surface retains its original gold color, indicating a negative result. This diagnostic assay tool is very sensitive and easily interpreted.
Bogart further discloses, in another embodiment of his invention (FIG. 17 of the aforementioned patent), the use of these substrates for enhanced fluorescence detection. After the analyte of interest has been bound to the surface by reaction with the specific binding partner in the receptive layer of the substrate coating, fluorescent label molecules may be attached to the analyte. In particular, the fluorescent molecules may be attached to any suitably selective and specific receptive material or reagent, such as a secondary antibody, and applied to the surface. The fluorescent labels are thus bound to the analyte of interest on the surface, if present, and immobilized to the surface through the analyte bridge. Directing light of an excitation wavelength onto the surface stimulates fluorescence of any of the label bound to the surface, thereby revealing the presence of the analyte of interest. Because the maximum fluorescence wavelength may not be shifted far enough from the excitation wavelength to be distinguished, the reflective substrate may have an antireflection layer whose thickness is selected to suppress reflection of the excitation wavelength, thereby reducing the background noise reaching the detector. Bogart states that the fluorescent signal generation is not dependent on the film thickness.
Even if the background noise is minimized (and even if the substrate is constructed so that the reflected or fluorescent sample signal stands out more clearly against the substrate's background by way of contrast), the maximum possible signal-to-noise ratio (SNR.sub.max) is still limited to: SNR.sub.max =S.sup.1/2. Though the fluorescence signal S might be increased by increasing the output power of the laser, reflected laser noise will also increase, with possibly little improvement in the resulting SNR.
An object of the present invention is to provide an improved sample substrate which provides increased sample excitation and fluorescence emission.